1. Field of the Invention
The present invention relates to a printing blanket in a seamless cylindrical shape which is particularly suitable for use in high-speed printing presses such as high-speed web offset printing press.
2. Description of the Prior Art
Examples of a printing blanket particularly suitable for the above-mentioned high-speed printing presses include one in a seamless cylindrical shape in the circumferential direction.
Such a printing blanket in a cylindrical shape is constructed by suitably laminating on an outer peripheral surface of a cylindrical sleeve mounted on a blanket cylinder of a printing press a porous and seamless compressible layer comprising an elastomer such as rubber, a non-stretchable layer formed by winding a non-stretchable thread in helical fashion in the circumferential direction, and the like, and laminating on the uppermost layer a seamless surface printing layer similarly comprising an elastomer such as rubber.
A so-called air-type printing blanket having a porous compressible layer inside the surface printing layer is lower in compressive stress in a nip deformed portion produced by being pressed against a plate cylinder, as compared with a solid-type printing blanket having no compressible layer, as indicated by a two-dot and dash line in FIG. 4, for example.
A variation from P.sub.a1 to P.sub.a2 of the compressive stress in a case where the amount of distortion in the nip deformed portion is changed from .epsilon..sub.1 to .epsilon..sub.2 is smaller than a variation from P.sub.s1 to P.sub.s2 of the compressive stress in the solid-type printing blanket, as shown in FIG. 4.
Therefore, the air-type printing blanket is high in impact absorbability, and is superior in the effect of preventing impact produced at the time of feeding gears of the printing press, for example, from affecting printing precision.
The solid-type printing blanket causes so-called bulge by stress concentrations on the surface printing layer in the nip deformed portion, which might result in inferior printing such as out of register due to expansion in the circumferential direction, inferior paper feeding, double, or deformation of a dot pattern (particularly, dot gain). On the other hand, the air-type printing blanket also has the effect of preventing the above-mentioned inferior printing because the compressible layer has the function of lowering stress concentrations on the surface printing layer.
The examples include a compressible layer having a closed cell structure in which voids are independent of each other, which is formed by (i) foaming matrix rubber composing the compressible layer by an expanding agent which is decomposed by heating to emit gas, or (ii) blending a hollow microsphere with matrix rubber, for example, and a compressible layer having an open cell structure in which voids connect with each other, which is formed by (iii) a so-called leaching method for dispersing in matrix rubber particles, such as common salt particles, extractable by a solvent (water in the case of the common salt particles) which does not affect rubber, vulcanizing the matrix rubber, and then extracting the particles.
In order to form the compressible layer, however, a lot of complicated steps are required even in the printing blanket having either one of the structures as described above. Therefore, the air-type printing blanket is low in productivity than the solid-type printing blanket.